Maternal and Child Health Bureau ~ 22
Accurately Weighing and Measuring: Technique
INTRODUCTION
With the availability of the WHO Growth Standards, it is an opportune time for all pediatric health care providers to re-evaluate the tools they use and the approach they have in their clinical setting for measurement, plotting and interpretation of growth charts.
This module reviews equipment for measuring and recording length, weight, and head circumference for infants, children and adolescents.
The information in this module is intended for the measurement of typically developing children. Another module provides information on the measurement techniques to be used when children have special physical considerations.
In this module ‘length’ refers to the measurement technique for infants. Length is measured in the recumbent position. ‘Stature’ refers to the child and adolescent ‘height’ measure. Stature is measured standing.
OBJECTIVES
· To present accurate techniques for measuring weight, length and head circumference for infants
· To present accurate techniques for measuring weight and stature for children and adolescents
TABLE OF CONTENTS
1. Importance of Reliable and Accurate Measurements
2. Three Components of Accurate Measuring
3. Weighing Infants
4. Measuring Infant Length
5. Measuring Head Circumference
6. Weighing Children and Adolescents
7. Measuring Child and Adolescent Stature
8. Which Units to Use?
9. Plotting Measurement Data
10. References
1. WHY IS IT IMPORTANT TO WEIGH AND MEASURE INFANTS, CHILDREN AND ADOLESCENTS ACCURATELY?
Accuracy is important in obtaining all pediatric size measurements because these measurements will be used as the basis of clinical assessment and to calculate Body Mass Index (BMI).
If growth is not proceeding as expected for an individual of a given age based on the size measures, then referral for additional evaluation may be necessary to address the concern.
The measurement process has two steps:
1. measure
2. record
If measures are in error, then the foundation of the growth assessment is also in error. It is important to have the date, age, and actual measurements recorded so the data may be used by others or at a later point in time.
· Many clinical decisions and clinical interventions are based on physical measurements
· Accurate and reliable physical measures are used to:
· monitor the growth of an individual
· detect growth abnormalities
· monitor nutritional status
· track the effects of medical or nutritional intervention
Parameters of Measurement Accuracy
To address quality assurance issues, there are two sets of numbers of interest.
The first set is the degree of refinement of a measure. That is, the degree to which a measure is recorded. For example, infant weight is recorded to 0.01 kg, 10 grams, or 1/2 ounce. If a newborn infant was weighed only to 0.1 kg, 100 grams, or 3 ounces, a rate of weight gain of less than 100 grams would not be reflected in the measure.
The tolerance of a measure is the difference between two measures that is accepted as reasonable accuracy. The tolerance of a measure is generally larger than the degree of refinement of a measure. For example, the weight of an infant is recorded as 3.12 kg and on re-measuring it is recorded as 3.13 kg. These measures are within an acceptable tolerance. If, however, the infant was weighed at 3.12 kg and a second measure was 3.2 kg, the infant should be re-weighed. If the third measure was recorded as 3.11 kg, the average of the two closest measures would be recorded.
The tolerance of a measure is generally larger for measures of older children and adolescents because small changes are less critical for the interpretation of growth.
2. THREE COMPONENTS OF ACCURATE MEASURING
Accurate weighing and measuring have three critical components. These are: technique, equipment, and trained measurers.
Technique: Standardized
Equipment: Calibrated, accurate
Trained measurers: Reliable, accurate
Appropriate technique for each measure must be utilized. The techniques should be very similar to those used to obtain data to develop the growth charts. These measures should be performed by a trained measurer so they are both accurate and reliable.
Accurate, calibrated equipment appropriate to the measurements being obtained is required. (Appropriate equipment is addressed in the Accurately Weighing and Measuring: Equipment module.)
There is a deceptive simplicity about the measurement of length or stature and weight. Many measurers believe the procedures to be so straightforward and obvious that they do not require any training to accurately perform the measures. However, standardization exercises have demonstrated that even experienced measurers can be inaccurate or even careless in performing weight and length or stature measurements.
The individual obtaining the measurements must understand the importance of reliable equipment, standardized technique, and the need for reproducible and accurate physical growth data.
Much of pediatric clinical assessment is based on the physical measurement data obtained and plotted on a growth chart. If this information is not reliable because of inadequate equipment, unacceptable technique or recording error, the data may lead to a clinical impression that is in error.
3. WEIGHING INFANTS: EQUIPMENT AND PREPARATION
The specific recommendations for equipment necessary for accurate and reliable weighing are presented in the companion Accurately Weighing and Measuring: Equipment module. It is assumed that the scale has appropriate precision of 0.01 kg or 1/2 oz. for weighing an infant and is calibrated.
It is important that the infant be weighed using procedures similar to those used to collect the data for constructing the charts. It is also important to use consistent procedures.
The NHANES weight data were collected from infants who were wearing clean disposable diapers. Infants in the WHO Multicentre Growth Reference Study were weighed nude. Weighing infants with too much clothing is one of the most frequent sources of error in infant weight measurements. This causes infants to be ‘weighed heavy’; an infant will appear to weigh more than he actually does.
/ q Infant is weighed nude or in a clean diaper ona calibrated beam or electronic scale
Weighing Infants: Procedures
It is desirable that two people be involved with infant weight measures. One measurer will weigh the infant (and protect the infant from harm ... falling, etc.) and read the weight as it is obtained. The other measurer will immediately note the measurement in the infant’s chart.
The infant’s clothing is removed and the infant is nude or wearing a clean, dry diaper.
Regardless of the type of infant scale used, the infant should be positioned in the center of the scale tray. Infants should be weighed to the nearest 0.01 kg or 1/2 oz.
The use of metric measures is encouraged when weighing infants, children, and adolescents in a clinical setting.
q Remove infant’s clothing and be sure thediaper is clean and dry
q Center the infant on the scale tray
q Weigh infant to nearest 0.01 kg or 1/2 oz /
Weighing Infants: Quality of Measurements
Record the weight as soon as it is completed. Then the infant should be re-positioned and the weight measurement repeated and noted in writing.
After the infant is removed from the scale tray, the weights should be compared and they should agree within 0.1 kg or 1/4 lb.
If the difference between the weights exceeds the tolerance limit of 0.1 kg or 1/4 lb, the infant should be re-positioned and reweighed a third time. Then record the average of the two weights in closest agreement.
Experienced measurers will be adept at handling infants and sensitive to their response to physical manipulation. Confidence and a sure manner will be reassuring to parents.
q Write the weight on the infant’s chart
q Reposition and repeat weighing the infant
q Compare weights
q Weight should agree within 0.1 kg or 1/4 lb. This is the tolerance of the measure.
Weighing Infants: Alternative Approach
Occasionally, an infant is too active or too distressed for an accurate weight measurement. If the infant is too active, the measurement may be postponed until later in the clinic visit when the infant may be more comfortable with the setting.
An alternative measurement technique may be used if an electronic scale is available. Have the parent stand on the scale, reset (tare) the scale to zero, then have the parent hold the infant and read the infant’s weight. Remember that many adult scales generally only weigh to the nearest 100 gm.
q If infant is too active, postpone the measure until later
q Have parent stand on scale, tare, then read infant weight
4. MEASURING INFANT LENGTH: EQUIPMENT AND PREPARATION
Length, measured in the recumbent position, is the correct linear measurement for infants younger than 24 months of age or children aged 24 to 36 months who cannot stand unassisted.
Accurate length measurement requires a calibrated length board with certain features for measuring length in the recumbent position.
The critical components of a length board are 1) a fixed headpiece and 2) a moveable footpiece which is perpendicular to the surface of the table that the length board is on. Length boards are described in detail in the Accurately Weighing and Measuring: Equipment module.
It is important that infants be measured using procedures similar to those used in developing the charts. The NHANES length data were collected on infants wearing clean disposable diapers or diapers and undershirts.
Length measurements for infants and young children should be obtained while the child is dressed in light underclothing or a diaper. The child's shoes must be removed. Hair ornaments should be removed from the top of the head.
/ q Length is measured with a suitablemeasuring board
q Use a calibrated length board with
a fixed headpiece and movable
footpiece which is perpendicular to
the surface of the table
q Measure infant without shoes and
wearing light underclothing or diaper
Measuring Infant Length: Procedures
The child should be placed on his back in thecenter of the length board so that the child is
lying straight and his shoulders and buttocks
are flat against the measuring surface. The
child's eyes should be looking straight up.
Both legs should be fully extended and the
toes should be pointing upward with feet flat
against the footpiece. /
Accurate length measurements require two measurers. One measurer holds the infant’s head, with the infant looking vertically upward and the crown of the head in contact with the headpiece in the Frankfort Horizontal Plane. The head of the infant is firmly but gently held in position. The measurer gently cups the infant’s ears while holding the head. Make sure the infant's chin is not tucked in against his chest or stretched too far back.
/ While the second measurer holds the infant's
head in the proper position, the measurer
aligns the infant’s trunk and legs, extends
both legs, and brings the footpiece firmly
against the heels. The measurer places one
hand on the infant’s knees to maintain full
extension of the legs. The infant’s toes are
pointing upward.
It is imperative that both legs be fully extended
for an accurate and reproducible length
measurement. If only one of the infant’s legs
is extended during the length measurement,
the measurement may be unreliable and
inaccurate. Correctly positioning the infant for
a length measurement generally cannot be
accomplished without two measurers. /
Parents may participate in the length measurement [between the two trained measurers] to provide reassurance and security to the infant.
Should length or stature be measured for a child aged 24 to 36 months? The best guideline is to think about the physical abilities of the child. Generally, if the child can stand unassisted and follow directions for proper positioning, a stature measure should be taken. However, if there are concerns about the child's growth and the previous measure was length, then length should be measured again. (See the CDC training materials: Case Example 5 for more information about transitions between charts.)
Maintaining a record of the child’s length on the "Birth to 36 months" chart may be helpful in circumstances where it is necessary to monitor small increments of growth.
What is the Frankfort horizontal plane?
The formal definition of the Frankfort horizontal plane is a line extending from the most inferior point of the orbital margin to the left tragion. The tragion is the deepest point in the notch superior to the tragus of the auricle.
Illustration © Nardella, M, Campo, L, Ogata, B, eds. Nutrition Interventions
for Children with Special Health Care Needs, Olympia, WA, State Department of Health, 2001. /
When the head is positioned correctly, the Frankfort horizontal plane is parallel to the fixed headpiece.
For length measures, the Frankfort plane is aligned perpendicular to the plane of the measuring table and parallel to the headpiece.
Measuring Infant Length: Quality of Measurements
The measurer at the feet should read aloud to the recorder the length measurement to the nearest 0.1 cm or 1/8 inch.
The length should be recorded on the data form as soon as it is completed.
Then the infant should be repositioned and the length measurement repeated and noted in writing.
After the infant is removed from the lengthboard, the length measurements should be compared and they should agree within 1 cm or 1/4 inch. This is the tolerance limit of this measure.
If the difference between the length measures exceeds the tolerance limit, the infant should be repositioned and remeasured a third time. The average of the two measures in closest agreement is recorded.
q Measure length to 0.1 cm or 1/8 inchq Record measurement on chart
q Reposition and remeasure infant
q Measurements should agree to 1 cm
or 1/4 inch -- the tolerance of
the measure /
Illustration © Nardella, M, Campo, L, Ogata, B,
eds. Nutrition Interventions for Children with
Special Health Care Needs, Olympia, WA,
State Department of Health, 2001.
5. MEASURING HEAD CIRCUMFERENCE: EQUIPMENT AND PREPARATION